Optical pickup device

ABSTRACT

An optical pickup device is applied to DVD recorders and includes a beam expander unit. A drive coil wound around a lens holder within the beam expander unit, is located in a magnetic field formed by a magnet. When an electric current is passed through the drive coil, the drive coil moves an expander second lens in a direction of an optical axis, receiving a force based on interaction between the electric current and the magnetic field. This changes a distance between an expander first lens and the expander second lens, and thus light emitted from the beam expander unit toward an object lens becomes diffused light or converged light. Consequently, the objective lens generates a spherical aberration in a reverse direction of a spherical aberration that has occurred owing to a deviation of a cover layer in film thickness from a reference value, which cancels the latter spherical aberration.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical pickup device. Morespecifically, the present invention relates to an optical pickup devicefor performing data recording/reproduction on an optical disk with acover layer formed on a signal recording layer.

2. Description of the Prior Art

In recent years, an optical disk has been widely used as a medium forrecording video data, audio data, computer data, etc. The need for anoptical disk with high recording density and high capacity has been moreand more increasing.

An optical disk has a cover layer through which light enters a signalrecording layer. Recording/reproduction is carried out by irradiatingthe light to the signal recording layer through the cover layer. Anobject lens is designed in such a manner that, when a film thickness ofthe cover layer meets a reference value (standard value in thespecification), a spherical aberration is a minimum on the signalrecording layer. Thus, a spherical aberration occurs if the thickness ofthe cover layer differs from the reference value, for example, if aplurality of signal recording layers exist on one side of the opticaldisk, or if the cover layer exhibits manufacturing variations in filmthickness.

According to the conventional optical pickup device disclosed inJapanese Patent Laying-open No. 2003-77142 (G11B 7/085), a beam expandercomposed of two convex lenses or one convex lens and one concave lens islocated between a collimator lens and an object lens in order to correctsuch a spherical aberration that occurs due to a deviation of the coverlayer in film thickness from the reference value. The beam expanderchanges a distance between the two lenses by driving one of the lenseswith a stepping motor so as to adjust the emitted light in such a mannerthat it becomes converged light or diffused light. If the film thicknessof the cover layer of the optical disk is small, for example, the beamexpander lets converged light into the object lens. As a result, aspherical aberration due to the thin cover layer is cancelled by aspherical aberration that has occurred on the object lens, which causesalmost no spherical aberration on the signal recording layer.

However, if the stepping motor for driving the lens in the beam expanderis to be provided within an optical pickup device, there is a problemwhere the optical pickup device capable of correcting a sphericalaberration becomes large in size.

SUMMARY OF THE INVENTION

It is therefore a primary object of the present invention is to providea novel optical pickup device.

It is another object of the present invention is to provide an opticalpickup device that is small in size and capable of correcting aspherical aberration.

It is still another object of the present invention is to provide anoptical pickup device that allows a control system to be simplified.

According to claim 1, an optical pickup device for datarecording/reproduction on an optical disk with a cover layer formed on asignal recording layer, comprises a light source, an object lens forfocusing light from the light source onto the signal recording layerthrough the cover layer, a beam expander unit including a fixed portionand a movable portion, provided between the light source and the objectlens, which converts light from the light source into any one ofdiffused light, converged light and collimated light and emits thelight, and a moving means that moves the movable portion in a directionof an optical axis by passing an electrical current through a drive coilcontained in the movable portion provided in a magnetic field.

In the present invention of claim 1, the drive coil in the movableportion of the beam expander unit is located within the magnetic field.Thus, when an electric current is passed through the drive coil, thedrive coil moves the movable portion in the direction of the opticalaxis, receiving a force based on interaction between the electriccurrent and the magnetic field. This changes a distance between twolenses contained in the beam expander unit, and the light emitted fromthe beam expander unit becomes diffused light or converged light. As aconsequence, the object lens generates a spherical aberration in areverse direction of a spherical aberration due to a deviation of thecover layer in film thickness from the reference value, therebycanceling the spherical aberration due to the deviation from thereference value. In this case, no stepping motor for moving the movableportion is required, which makes it possible to downsize the opticalpickup device capable of correcting a spherical aberration.

Also, as compared to a case where the stepping motor is employed, thereis no need for four or more signal lines for controlling the steppingmotor and two to four signal lines for controlling a limit switch forthe detection of position of a movable portion. All required signallines are only two for controlling the drive coil. With this, thecontrol system of the optical pickup device can be simplified.

An optical pickup device of claim 2 is dependent on claim 1. The drivecoil is wound in a direction orthogonal to the direction of the opticalaxis. The magnet is positioned in the fixed portion so that a magneticfield may be generated in a direction orthogonal to both the directionof an electric current passed through the drive coil and the directionof the optical axis. In this case, when an electric current is passedthrough the drive coil, the drive coil receives a force in the directionof the optical axis based on interaction between the electric currentand the magnetic field. When the drive coil has received the force, themovable portion moves in the direction of the optical axis, and thelight incident onto the object lens becomes diffused light or convergedlight. As a consequence, it is possible to suppress the occurrence of aspherical aberration.

An optical pickup device of claim 3 is dependent claim 1 or 2. The beamexpander unit further comprises a restoration means for restoring themovable portion to a position before having been moved by the drivecoil. In this case, the movable portion that has been moved in thedirection of the optical axis can be restored by the restoration meansto the pre-movement position, which eliminates the need for providingthe limit switch for detecting the position of the movable portionwithin the optical pickup device. This allows the optical pickup deviceto be made more compact.

An optical pickup device of claim 4 is dependent on claim 3. Therestoration means includes two spiral leaf springs that make no contactwith each other. The two spiral leaf springs are electrically connectedwith the both ends of the drive coil, respectively. In this case, themovable portion is restored by the two spiral leaf springs to thepre-movement position. Making the leaf springs spiral decreases a loadin the movement direction, which improves the movable portion insensitivity. Additionally, since the two spiral leafs do not touch witheach other, it is possible to electrically connect the both ends of thedrive coil wound around the movable portion to the two spiral leafsprings, respectively, and supply an electric current to the drive coilvia the spiral leaf springs.

An optical pickup device of claim 5 is dependent on any one of claims 1to 4, and further comprises a collimator lens provided between the lightsource and the beam expander unit, which converts light from the lightsource into collimated light. In this case, the light from the lightsource is converted by the collimator lens into collimated light and isentered into the beam expander unit. This makes it easy to makeadjustments for converting the incident light into diffused light,converged light or collimated light.

An optical disk recording/reproduction apparatus of claim 6 includes anoptical pickup device according to one of claim 1 to 5, and furthercomprises a temperature sensor for measuring an ambient temperature ofthe light source, and a first control means for controlling the movingmeans based on a result measured by the temperature sensor. In thiscase, a wavelength of light from the light source varies depending onthe ambient temperature of the light source. In addition, as arefractive index changes according to the fluctuations of the lightwavelength, the virtual film thickness of the cover layer varies,resulting in a spherical aberration. Therefore, the first control meanscontrols the moving means in such a manner as to move the movableportion of the beam expander unit along the optical axis, based on theambient temperature of the light source measured by the temperaturesensor, thereby suppressing the occurrence of a spherical aberration.

An optical disk recording/reproduction apparatus of claim 7 is dependingon claim 5, and further comprises an output sensor for measuring outputof the light source, and a second control means for controlling themoving means based on a result measured by the output sensor. In thiscase, as the output of the light source changes, the wavelength of lightemitted from the light source varies as well. Also, as a refractiveindex changes according to the fluctuations of light wavelength, thevirtual film thickness of the cover layer varies, resulting in aspherical aberration. Therefore, the second control means controls themoving means in such a manner as to move the movable portion along theoptical axis, based on the output of the light source measured by theoutput sensor, thereby suppressing the occurrence of a sphericalaberration.

An optical disk recording/reproduction apparatus of claim 8 is dependingon claim 6 or 7, and further comprises a detection means for detecting aposition of the object lens along the optical axis, and a third controlmeans for controlling the moving means based on a result detected by thedetection means. In this case, the detection means firstly obtains theposition of the object lens along the optical axis. Next, the thirdcontrol means controls the moving means based on the obtained positionof the object lens. As a result, the movable portion moves along theoptical axis, suppressing the occurrence of a spherical aberration.

The above described objects and other objects, features, aspects andadvantages of the present invention will become more apparent from thefollowing detailed description of the present invention when taken inconjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1(A) is a perspective view of an optical pickup device of oneembodiment of the present invention. FIG. 1(B) is a front view of theoptical pickup device shown in FIG. 1(A). FIG. 1(C) is a side view ofthe optical pickup device shown in FIG. 1 (A).

FIG. 2(A) is a sectional view showing a relationship between lightemitted from an object lens of the optical pickup device shown in FIG.1(A) and a first signal recording layer of the optical disk. FIG. 2(B)is another sectional view showing a relationship between the lightemitted from the object lens of the optical pickup device shown in FIG.1(A) and the first signal recording layer of the optical disk.

FIG. 3(A) is a sectional view showing a relationship between the lightemitted from the object lens of the optical pickup device shown in FIG.1(A) and a second signal recording layer of the optical disk. FIG. 3(B)is another sectional view showing a relationship between the lightemitted from the object lens of the optical pickup device shown in FIG.1(A) and the second signal recording layer of the optical disk.

FIG. 4(A) is a sectional view showing a relationship between the lightemitted from the object lens of the optical pickup device shown in FIG.1(A) and the first signal recording layer and second signal recordinglayer of the optical disk. FIG. 4(B) is another sectional view showing arelationship between the light emitted from the object lens of theoptical pickup device shown in FIG. 1(A) and the first signal recordinglayer and second signal recording layer of the optical disk. FIG. 4(C)is a further sectional view showing a relationship between the lightemitted from the object lens of the optical pickup device shown in FIG.1(A) and the first signal recording layer and second signal recordinglayer of the optical disk.

FIG. 5 is an illustrative view showing a relationship between a positionof an expander second lens of the optical pickup device shown in FIG.1(A) and light incident on the object lens.

FIG. 6 is a graph indicating an effect of correction made by the opticalpickup device shown in FIG. 1(A).

FIG. 7 is an illustrative view showing a structure of a beam expanderunit of the optical pickup device shown in FIG. 1(A).

FIG. 8(A) is an illustrative view showing one of spiral leaf springs ofthe optical pickup device shown in FIG. 1(A). FIG. 8(B) is anillustrative view showing the other spiral leaf spring of the opticalpickup device shown in FIG. 1(A). FIG. 8(C) is an illustrative viewshowing a state where the spiral leaf spring shown in FIG. 8(A) and thespiral leaf spring shown in FIG. 8(B) are combined with each other.

FIG. 9(A) is a plane view showing the spiral leaf springs in the beamexpander shown in FIG. 7. FIG. 9(B) is a sectional view taken along aline IXB-IXB shown in FIG. 9(A).

FIG. 10 is an illustrative view showing a principle of operation of theoptical pickup device shown in FIG. 1(A).

FIG. 11 is a block diagram showing an optical diskrecording/reproduction apparatus including the optical pickup deviceshown in FIG. 1(A).

FIG. 12 is a flowchart showing an operation of the optical diskrecording/reproduction apparatus shown in FIG. 11.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1(A) to FIG. 1(C), an explanation will be given belowregarding an optical pickup device 10 of an embodiment of the presentinvention. FIG. 1(A) to FIG. 1(C) show a perspective view, plan view andside view of the optical pickup device 10, respectively.

The optical pickup device 10 includes a semiconductor laser 12 as alight source. A laser beam of linearly polarized light emitted from thesemiconductor laser 12 passes through a polarization beam splitter 14and then is converted into circularly polarized light by a ¼ wavelengthplate 16.

The laser beam converted into circularly polarized light is collimatedby a collimator lens 18 and enters a beam expander unit 20. The beamexpander unit 20 includes an expander first lens 22 as concave lens andan expander second lens 24 as convex lens. The expander second lens 24moves in an X direction along an optical axis of the laser beam. Thedetailed structure of the beam expander unit 20 will be described later.

After passing through the beam expander unit 20, the laser beam isreflected by a 45-degree reflecting mirror 26 in a +Z direction towardwhich the object lens 28 is located, and is focused by the object lens28 onto a signal recording layer of an optical disk 30.

The laser beam of circularly polarized light reflected by the signalrecording layer, passes through the object lens 28 and is reflected bythe 45-degree reflecting mirror 26. The reflected laser beam passesthrough the beam expander unit 20 and the collimator lens 18, and isconverted by the ¼ wavelength plate 16 into a laser beam of linearlypolarized light. The converted laser beam of linear polarization isrotated 90 degrees with respect to a direction of polarizationimmediately after it has been emitted from the semiconductor laser 12.The laser beam converted into the linearly polarized light is reflectedby the polarization beam splitter 12 in a +Y direction forming an angleof 90 degrees with respect to the incident direction.

The reflected light passes through a condensing lens 32 and acylindrical lens 34 that causes astigmatism so as to control a focusservo, and enters a photodetector 36. The photodetector 36 includes aphotodiode and outputs a signal corresponding to the strength of theincident light.

In reference to FIG. 2(A) to FIG. 4(C), an explanation will be providedbelow as to a relationship between a spherical aberration resulting froma deviation of a cover layer of the optical disk 30 from a referencevalue in film thickness and the light incident on the object lens 28.

Firstly, referring to FIG. 2(A), a description will be given regarding acase where the object lens 28 is designed so as to focus the collimatedincident light onto a first signal recording layer 30 b near a disksurface 30 a. In such a case, when the collimated light enters theobject lens 28, the spherical aberration of the light focused onto thefirst signal recording layer 30 b becomes minimized.

In order to focus the collimated incident light by using the object lens28 onto a second signal recording layer 30 c located in a deeperposition than the first signal recording layer 30 b, it is necessary tolet the incident light pass through not only a cover layer 38 a but acover layer 38 b as well. Consequently, there occurs a sphericalaberration in the light focused onto the second signal recording layer30 c. With that, as shown in FIG. 2(B), if the diffused light enters theobject lens 28, there occurs a spherical aberration in the light emittedfrom the object lens 28 that cancels the above-mentioned sphericalaberration almost completely. This makes it possible to suppress aspherical aberration of the light focused on the second signal recordinglayer 30 c.

Next, referring to FIG. 3(A), an explanation will be provided below asto a case where the object lens 28 is designed so as to focus thecollimated incident light onto the second signal recording layer 30 cdistant from the disk surface 30 a. In such a case, when the collimatedlight enters the object lens 28, the spherical aberration of the lightfocused onto the second signal recording layer 30 c becomes minimized.

If the collimated incident light is focused by using the object lens 28onto the first signal recording layer 30 b located in a shallowerposition than the second signal recording layer 30 c, the incident lightpasses through only the cover layer 38 a. Consequently, there occurs aspherical aberration in the light focused onto the first signalrecording layer 30 b. With that, as shown in FIG. 3(B), if the diffusedlight enters the object lens 28, there occurs a spherical aberration inthe light emitted from the object lens 28 that cancels theabove-mentioned spherical aberration almost completely. This makes itpossible to suppress a spherical aberration of the light focused on thefirst signal recording layer 30 b.

Additionally, in reference to FIG. 4(A), a description will be presentedbelow regarding a case where the object lens 28 is designed so as tofocus the collimated incident light onto a middle position 30 d of thecover layer 38 b sandwiched between the first signal recording layer 30b and the second signal recording layer 30 c. In such a case, when thecollimated light enters the object lens 28, the spherical aberration ofthe light focused onto the middle position 30 d of the cover layer 38 bbecomes minimized.

If the collimated incident light is focused by use of the object lens 28onto the first signal recording layer 30 b located in a shallowerposition than the middle position 30 d of the cover layer 38 b, theincident light passes through only the cover layer 38 a. Consequently,there occurs a spherical aberration on the first signal recording layer30 b. With that, as shown in FIG. 4(B), if the converged light entersthe object lens 28, there occurs a spherical aberration in the lightemitted from the object lens 28 that cancels the above-mentionedspherical aberration almost completely. This makes it possible tosuppress a spherical aberration in the light focused on the first signalrecording layer 30 b.

In order to focus the collimated incident light by using the object lens28 onto the second signal recording layer 30 c located in a deeperposition than the middle position 30 d of the cover layer 38 b, it isnecessary to let the incident light pass through not only the coverlayer 38 a but the cover layer 38 b as well. Consequently, there occursa spherical aberration in the light focused onto the second signalrecording layer 30 c. With that, as shown in FIG. 4(C), if the diffusedlight enters the object lens 28, there occurs a spherical aberration inthe light emitted from the object lens 28 that cancels theabove-mentioned spherical aberration almost completely. This makes itpossible to suppress a spherical aberration of the light focused on thesecond signal recording layer 30 c.

As can be seen from the above explanations, it is possible to suppress aspherical aberration of the light focused onto the first signalrecording layer 30 b or the second signal recording layer 30 c byconverging or diffusing the light incident on the object lens 28.

Incidentally, in a case where the cover layers 38 a and 38 b of theoptical disk 30 deviate from the reference value in film thickness dueto manufacturing variations in film thickness of the optical disk 30, aspherical aberration can be also suppressed in the same way.

Moreover, if there is any change in the output of the semiconductorlaser 12 as a light source and the ambient temperature of the same,etc., the wavelength of laser beam also varies slightly with that. Atthis time, there are some fluctuations in the refractive indexes of thecover layers 38 a and 38 b corresponding to the wavelength of laserbeam, which lead to variations in the virtual film thicknesses of thecover layers 38 a and 38 b. This induces a spherical aberration. In thiscase as well, the spherical aberration can be suppressed in the same wayas mentioned above.

Next, referring to FIG. 5, it will be explained below that the lightincident on the object lens 28 can be converted into collimated light,diffused light or converged light by changing the distance between twolenses contained in the beam expander unit 20.

In this embodiment, the expander first lens 22 (on the side of thecollimator lens 18) is taken as a concave lens and the expander secondlens 24 (on the side of the object lens 28) as a convex lens. Thesemiconductor laser (light source) 12, the collimator lens 18, theexpander first lens 22, and the object lens 28 are fixed in theirrespective positions. Only the expander second lens 24 can move in thedirection of the optical axis. Also, “a” denotes a focal length of theobject lens 28 on an emitting side (rear-side focal length) and “b”indicates a focal length of the object lens 28 on an incidence side(front-side focal length).

A middle drawing in FIG. 5 indicates that the film thickness of thecover layer of the optical disk 30 meets the reference value. At thistime, the distance between the expander first lens 22 and the expandersecond lens 24 is a distance at which the collimated light emitted fromthe collimator lens 18 is diffused by the expander first lens 22 and iscollimated again by the expander second lens 24. Thus, the light emittedfrom the expander second lens 24 is collimated light.

An upper drawing in FIG. 5 indicates that the film thickness of thecover layer of the optical disk 30 is larger than the reference value.At this time, the expander second lens 24 has moved nearer to the lightsource 12 as compared to the case of the middle drawing, so the lightemitted from the expander second lens 24 is diffused light.

A lower drawing in FIG. 5 indicates that the film thickness of the coverlayer of the optical disk 30 is smaller than the reference value. Atthis time, the expander second lens 24 has moved nearer to the objectlens 28 as compared to the case of the middle drawing, so the lightemitted from the expander second lens 24 is converged light.

Referring to FIG. 6, an explanation will be given below about aspherical aberration that occurs if the film thickness of the coverlayer of the optical disk 30 has changed within a range from 70 μm to130 μm, and about a result on a simulation of correction made to thisspherical aberration. Here, the reference value of the film thickness ofthe cover layer is 100 μm, and a numerical opening NA of the object lensis 0.85.

For example, if the film thickness of the cover layer deviates by ±25 μmfrom the reference value of 100 μm, that is, if the film thickness ofthe cover layer is 125 μm or 75 μm, as can be seen from FIG. 6, theamount of a spherical aberration increases up to about 0.25 λ (“λ”denotes the wavelength of light). To achieve favorablerecording/reproduction characteristics, it is generally said that theamount of the spherical aberration generated by the optical pickupdevice 10 must be held at 0.07 λ or less. Accordingly, if the amount ofthe spherical aberration is 0.25 λ, no favorable recording/reproductioncharacteristics can be obtained.

Thus, it becomes possible to keep the amount of spherical aberration at0.01 λ or less that hardly affects the recording/reproductioncharacteristics of the optical pickup device 10 by moving the expandersecond lens 24 in the direction of the optical axis and enteringdiffused light or converged light into the object lens 28.

Next, referring to FIG. 7, a description will be provided below on thestructure of the beam expander unit 20. The beam expander unit 20includes a movable portion 20 a and a fixed portion 20 b. The movableportion 20 a includes a lens holder 40 with the expander second lens 24fixed as a convex lens, the drive coil 44 wound around the outerperiphery of the lens holder 40 in such a manner as to be orthogonal tothe optical axis, and the two spiral leaf springs 46 a and 46 b. One endof the lens holder 40 has the drive coil 44 wound around it, and theother end has a protrusion whose diameter is larger than the one end andis divided into quarters by notches. The both ends of the drive coil 44are electrically connected to the spiral leaf springs 42 a and 42 b,respectively. Details on the method of combining the spiral leaf springs42 a and 42 b will be described later.

The fixed portion 20 b includes a yoke 50. A ring-shaped magnet 46 ispositioned on an inner wall of an opening formed in the yoke 50, and theexpander first lens 22 is fixed to the middle of the opening of the yoke50. The lens holder 40 of the movable portion 20 a is stored in spacebetween the magnet 46 and the expander first lens 22 in such a manner asto be movable in the direction of the optical axis. The yoke 50constitutes a magnetic circuit together with the magnet 46, and performsa function of increasing the strength of a magnetic field generated bythe magnet 46.

The yoke 50 is insulated from the spiral leaf springs 42 a and 42 b by aspacer 48 consisted of an insulating material. The spacer 48 iscylindrical in form, and the diameters of its both ends are smaller thanthe diameter of the middle part thereof. One of the ends is insertedinto a hole 52, and the other end is inserted into a hole formed in thespiral leaf springs 42 a and 42 b at two positions each. Consequently,the yoke 50 is insulated from the spiral leaf springs 42 a and 42 b bythe middle part of the spacer 48. The method of fixing the spiral leafsprings 42 a and 42 b to the fixed portion 20 b will be described laterin detail.

If the magnet 46 is ring-shaped, it is possible to reduce components innumber. However, there is a problem where it is difficult to manufacturethe magnet 46 due to its small size. On this account, the magnet 46 maybe arranged on the inner wall of the opening in a form divided into aplurality of parts, so that it takes on a fan shape as viewed from thedirection of the optical axis.

Referring to FIG. 8(A) to FIG. 8(C), a description will be providedbelow on the forms of the spiral leaf springs 42 a and 42 b and themethod of combining them. As can be seen from FIG. 8(A), the spiral leafspring 42 b has four circular arcs of different lengths arranged in aswirling manner with space wider than the width of the arc between them.These arcs are connected not so as to separate from each other. The arcon the outermost perimeter has two holes. When the other end of thespacer 48 is inserted into the hole, the spiral leaf spring 42 b isfixed to the fixed portion 20 b of the beam expander 20.

As is apparent from FIG. 8(B), the spiral leaf spring 42 a is ofidentical shape with the spiral leaf spring 42 b. Therefore, as shown inFIG. 8(C), when the spiral leaf spring 42 a and the spiral leaf spring42 b are combined with a rotation of 180 degrees from each other on thesame plane, each of the arcs of the spiral leaf spring 42 a ispositioned between the arcs of the spiral leaf spring 42 b. In this way,since the spiral leaf springs 42 a and 42 b are combined without contactbetween them, it is possible to supply an electric current to the drivecoil 44 via the spiral leaf springs 42 a and 42 b.

Referring to FIG. 9(A) and FIG. 9(B), the structure of the beam expanderunit 20 will be described below in more detail. FIG. 9(A) is anillustrated view of the beam expander unit 20 as seen from the +Xdirection (the direction perpendicular to the surface of paper sheetbearing FIG. 9 from the front side toward the back side). The arcs onthe outermost periphery of the spiral leaf springs 42 a and 42 b arefixed to the yoke 50 by the spacer 48. The arcs on the innermostperiphery are fixed with an adhesive to a YZ surface located at the baseof the protrusion formed in the lens holder 40. Incidentally, in FIG.9(A), some parts of the arcs on the innermost periphery can be seenthrough the four notches of the lens holder 40.

In this way, the spiral leaf springs 42 a and 42 b are spiral springs inwhich one end is fixed to the yoke 50 of the fixed portion 20 b and theother end to the lens holder 40 of the movable portion 20 a. This makesit possible to move the movable portion 20 a in the direction of theoptical axis, without having to apply a strong force to the movableportion 20 a. As a consequence, the movable portion 20 a can be improvedin sensitivity. In this case, the movable portion 20 a of the opticalpickup device 10 can be shifted by up to ±1 mm in the direction of theoptical axis from a state where no force is applied to it. Additionally,a spherical aberration due to deviations of the cover layers in filmthickness from the reference value can be cancelled by moving themovable portion 20 a by ±0.6 mm to 0.7 mm in the direction of theoptical axis. As a consequence, the use of the spiral leaf springs 42 aand 42 b makes it possible to cancel the spherical aberration due todeviations of the cover layers in film thickness from the referencevalue.

In addition, if the power switch is suddenly turned off because of apower failure etc., the movable portion 20 a is restored to the originalpoint by the restoring force of the spiral leaf springs 42 a and 42 b.Thus, it is unnecessary to provide a limit switch for detecting theposition of the movable portion 20 a when the power switch has beenturned on again afterward. This allows the optical pickup device 10 tobe more miniaturized.

Moreover, FIG. 9(B) is a sectional view of the beam expander unit 20taken along a line IXB-IXB as seen from the +Y direction. As is apparentfrom FIG. 9(B), the lens holder 40 is mounted in such a manner as to bemovable within the yoke 50 in the X direction (the direction of theoptical axis). The expander second lens 24 is fixed into the lens holder40, and the drive coil 44 is wound around the outer periphery of thelens holder 40. Also, the expander first lens 22 is fixed to the centerof the opening of the yoke 50, and the magnet 46 is located on the innerwall of the opening so as to surround the expander first lens 22.

With reference to FIG. 10, a driving principle of the beam expander unit20 will be described below. The magnet 46 is positioned in such a manneras to surround the drive coil 44 of the movable portion 20 a. When theinner wall of the magnet 46 is at the N Pole and the outer wall of thesame is at the S Pole, the direction of a magnetic field generated bythe magnet 46 is a direction radiating out from the center of the magnet46 in the YZ plane. In addition, the drive coil 44 is wound around theoptical axis in the direction orthogonal to the optical axis.

When a clockwise electric current is passed through the drive coil 44, aforce is exerted on the drive coil 44 in the +X direction perpendicularboth to the direction of the current and the direction of the magneticfield (direction perpendicular to the surface of paper sheet bearingFIG. 10 from the back side toward the front side) according to Fleming'sleft-hand rule. As a result, the distance between the expander secondlens 24 and the expander first lens 22 is longer, and the light emittedfrom the expander second lens 24 is converged light.

On the other hand, when a counter-clockwise electric current is passedthrough the drive coil 44, a force is exerted on the drive coil 44 inthe −X direction perpendicular both to the direction of the current andthe direction of the magnetic field (direction perpendicular to thesurface of paper sheet bearing FIG. 10 from the front side toward theback side) according to Fleming's left-hand rule. As a result, thedistance between the expander second lens 24 and the expander first lens22 become shorter, and the light emitted from the expander second lens24 is diffused light.

The spherical aberration on the first signal recording layer 30 b orsecond signal recording layer 30 c of the optical disk 30 can besuppressed in the following way. A clockwise current orcounter-clockwise current is passed through the drive coil 44 to movethe movable portion 20 a in the +X direction or the −X direction. Inother words, the drive coil 44 is positioned within the movable portion20 a of the beam expander unit 20, and the movable portion 20 a is movedin the direction of the optical axis by mutual interaction between thecurrent passed through the drive coil 44 and the magnetic field, whichleads to a change in the distance between the expander second lens 24and the expander first lens 22. As a consequence, a spherical aberrationis generated and makes it possible to cancel the spherical aberrationthat has occurred on the first signal recording layer 30 b or the secondsignal recording layer 30 c. Therefore, since there is no need toprovide a stepping motor for moving the movable portion 20 a within theoptical pickup device 10, the optical pickup device 10 can be madesmaller in size.

Furthermore, if no stepping motor is used, it becomes unnecessary tohave four or more signal lines for controlling the stepping motor andtwo to four signal lines for controlling the limit switch for thedetection of the position of the movable portion 20 a. All newlyrequired signal lines are only two ones for controlling the drive coil44, which allows the control system to be simplified.

Although the expander first lens 22 is taken as a concave lens and theexpander second lens 24 as a convex lens in the optical pickup device10, the expander first lens 22 may be a convex lens and the expandersecond lens 24 a concave lens instead. Alternatively, both the expanderfirst lens 22 and the expander second lens 24 may be a concave lens.

Next, referring to FIG. 11, a description will be given below as to anoptical disk recording/reproduction apparatus 60, such as DVD recorders.The optical disk recording/reproduction apparatus 60 includes theoptical pickup device 10, and further comprises a signal generationcircuit 62 and a CPU 72.

The optical pickup device 10 includes the semiconductor laser 12, thebeam expander unit 20, the object lens 28, the photodetector 36, a laserdrive circuit 64, and an object lens actuator 70. The optical pickupdevice 10 further comprises a temperature sensor 74 for measuring anambient temperature of the semiconductor laser 12, a front monitor diode68 for measuring output of the semiconductor laser 12. The temperatureand output measured by these components are supplied to the CPU 72.

Also, the signal generation circuit 62 generates a focus error signal, atracking error signal, an RF signal, etc. based on an output signal fromthe photodetector 36, and provides the CPU 72 with those generatedsignals.

When detecting that a laser beam is not focused on the signal recordinglayer of the optical disk 30 based on the focus error signal, the CPU 72drives the object lens actuator 70 to move the object lens 28 in thedirection of the optical axis, thereby focusing on the signal recordinglayer. The CPU 72 then calculates the position of the focused objectlens 28 along the optical axis, based on a focus drive voltage and afocus voltage sensitivity of the object lens actuator 70. The positionof the object lens 28 is here calculated because, if the object lens 28changes its position, the distance between the object lens 28 and theexpander second lens 24 varies as well, which affects the generation ofa spherical aberration.

Likewise, when detecting based on the tracking error signal that a laserbeam is out of a track of the optical disk 30, the CPU 72 drives theobject lens actuator 70 to move the object lens 28 in parallel with theprinciple surface of the optical disk 30 so that the laser beam isalways irradiated onto the track of the optical disk 30.

In addition, the CPU 72 controls the beam expander unit 20 and correctsa spherical aberration, based on the RF signal generated by the signalgeneration circuit 62, the ambient temperature of the semiconductorlaser 12 measured by the temperature sensor 74, the output of thesemiconductor laser 12 measured by the front monitor diode 68, and thecalculated position of the object lens 28. The ambient temperature andoutput of the semiconductor laser 12 here are required because, if thewavelength of the laser beam varies with a change in the ambienttemperature or output of the semiconductor laser 12, the refractiveindex of the cover layer of the optical disk 30 corresponding to thewavelength also changes, which leads to an alteration in the virtualfilm thickness of the cover layer with the occurrence of a sphericalaberration. Additionally, the position of the object lens 28 isnecessary because, if the object lens 28 is moved along the focusdirection to focus on the optical disk 30 when the optical disk 30 iscurved, the distance between the object lens 28 and the expander secondlens 24 changes accordingly.

The CPU 72 also controls the semiconductor laser drive circuit 64 inorder to stabilize the output of the semiconductor laser 12 based on theoutput value of the semiconductor laser 12 output from the front monitordiode 68, and to generate a strong pulse laser beam required forrecoding data on the optical disk 30.

Next, in reference to FIG. 12, an explanation will be given belowregarding a process flow of the CPU 72 for adjusting the position of thebeam expander unit 20. Firstly, in a step S1, it is determined whetheror not that the signal recording layer for performing datarecording/reproduction is a layer 0 as the first signal recording layer30 b of the optical disk 30. If “YES” has been determined, the signalrecording layer for performing data recording is the layer 0. In a stepS3, therefore, a coarse adjustment is made to the position of theexpander second lens 24 in the beam expander unit 20 with respect to thelayer 0. On the contrary, if “NO” has been determined, the layer forperforming data recording is a layer 1 as the second signal recordinglayer 30 c. In a step S5, therefore, a coarse adjustment is made to theposition of the expander second lens 24 with respect to the layer 1. Byperforming the step S3 or the step S5, the expander second lens 24 canbe moved to a tentative position along the optical axis so as to cancelthe spherical aberration almost completely.

Next, in a step S7, the resulting spherical aberration is determined onthe basis of the ambient temperature of the semiconductor laser 12, theoutput of the semiconductor laser 12 and the position of the object lens28. Based on the ambient temperature of the semiconductor laser 12measured by the temperature sensor 74, a spherical aberration ΔX_(T) iscalculated by the following equation (1) and equation (2).Δλ_(T) =A _(T) ×ΔT  (1)where Δλ_(T): Amount of wavelength change

ΔT: Amount of temperature change

A_(T): Coefficient varying by the film thickness of the cover layer andthe wavelength of laser beamΔX _(T) ≈B _(T)×λ_(T) =A _(T) B _(T) ×ΔT  (2)where ΔX_(T): Amount of a spherical aberration

B_(T): Coefficient varying by the film thickness of the cover layer andthe wavelength of laser beam

Likewise, a spherical aberration ΔX_(p) is calculated by the followingequation (3) and equation (4), on the basis of the output of thesemiconductor laser 12 measured by the front monitor diode 68.Δλ_(P) =A _(P) ×ΔP  (3)where Δλ_(p): Amount of wavelength change

ΔP: Amount of temperature change

A_(p): Coefficient varying by the film thickness of the cover layer andthe wavelength of laser beamΔX _(p) ≈B _(p)×λ_(p) =A _(p) B _(p) ×ΔP  (4)where ΔX_(p): Amount of a spherical aberration

B_(p): Coefficient varying by the film thickness of the cover layer andthe wavelength of laser beam

Moreover, a spherical aberration ΔX_(S1) or ΔX_(S2) is determined by thefollowing equation (5) to equation (9), on the basis of the position ofthe object lens 28.α=1/((1/f _(b2))+(1/(L1−f _(b1) +ΔB)))  (5)where f_(b1): Focal length of the beam expander first lens 22 (a minusvalue in the case of the convex lens)

f_(b2): Focal length of the beam expander second lens 24

ΔB: Amount of movement of the collimator lens 18 from the referenceposition (the position where the laser beam emitted from the collimatorlens 18 is collimated). ΔB on the object lens 28 side is positive.a=L2−α+ΔOL  (6)where L2: Distance between the object lens 28 and the beam expandersecond lens 24

ΔOL: Amount of movement of the object lens 28 from the referenceposition (the position of the object lens 28 where the laser beam isfocused on the signal recording layer meets the reference value). ΔOL onthe optical disk 30 side is positive.b=1/((1/f _(OL))+(1/a))  (7)where f_(OL): Focal length of the object lens 28Accordingly, a spherical aberration ΔX_(S1) in the case where thediffused light is incident on the object lens 28, is determined by thefollowing equation (8).ΔX _(S1) =C×(b/a)+E  (8)where C: Negative coefficient varying by the film thickness of the coverlayer and the wavelength of laser beam

E: Coefficient varying by the film thickness of the cover layer and thewavelength of laser beam

Furthermore, a spherical aberration ΔX_(S2) in the case where theconverged light is incident on the object lens, is determined by thefollowing equation (9).ΔX _(S2) =D×(b/a)+E  (9)where D: Positive coefficient varying by the film thickness of the coverlayer and the wavelength of laser beam

In a step S9, the expander second lens 24 is further moved by makingfine adjustments from the tentative position obtained in the step S3 orstep S5 in order to correct the spherical aberration obtained in thestep S7. In this way, after the position of the expander second lens 24has been determined, trial recording/reproduction of the optical disk 30is performed in a step S11.

In a step S13, it is determined whether the recording/reproduction inthe step S11 meets optimal conditions. This determination is made, forexample, according to whether or not the reproduced result of datarecorded in the step S11 is below a predetermined error rate.

If “NO” has been determined, the position of the expander second lens 24is shifted a little in a step S15, and the process returns to the stepS11.

On the contrary, if “YES” has been determined, recoding/reproduction isperformed on the optical disk 30 in a step S17.

Incidentally, in the step S1 of this process flow, it is determinedwhich of the two layers is the signal recording layer. However, theprocess flow can be also applied to the case where there exists three ormore signal recording layers and the case where the cover layer of theoptical disk 30 varies in film thickness.

Although the present invention has been described and illustrated indetail, it is clearly understood that the same is by way of illustrationand example only and is not to be taken by way of limitation, the spiritand scope of the present invention being limited only by the terms ofthe appended claims.

1. An optical pickup device for data recording/reproduction on anoptical disk with a cover layer formed on a signal recording layer,comprising: a light source; an object lens for focusing light from saidlight source onto said signal recording layer through said cover layer;a beam expander unit including a fixed portion and a movable portion,provided between said light source and said object lens, for convertinglight from said light source into any one of diffused light, convergedlight and collimated light and emits the light; and a mover for movingsaid movable portion in a direction of an optical axis by passing anelectrical current through a drive coil contained in said movableportion provided in a magnetic field, wherein said beam expander unitfurther comprises a restorator for restoring said movable portion to aposition before having been moved by said drive coil, and wherein saidrestorator includes two spiral leaf springs that are coplanar with armsthereof interleaved and that make no contact with each other, and saidtwo spiral leaf springs are electrically connected with the both ends ofsaid drive coil, respectively.
 2. An optical pickup device according toclaim 1, wherein said drive coil is wound in a direction orthogonal tosaid direction of the optical axis, and said magnet is positioned insaid fixed portion so that a magnetic field may be generated in adirection orthogonal to both the direction of an electric current passedthough said drive coil and the direction of the optical axis.
 3. Anoptical pickup device according to claim 1 or 2, further comprising acollimator lens provided between said light source and said beamexpander unit, for converting light from the light source intocollimated light.
 4. An optical disk recording/reproduction apparatusincluding an optical pickup device according to claim 1, furthercomprising: a detector for detecting a control factor based on whichmovement of said movable portion by said mover is to be controlled; anda controller for controlling said mover based on said control factordetected by said detector.
 5. An optical disk recording/reproductionapparatus including an optical pickup device according to claim 4,wherein said detector includes a temperature sensor for measuring anambient temperature of said light source as said control factor, andsaid controller controls said mover based on a result measured by saidtemperature sensor.
 6. An optical disk recording/reproduction apparatusincluding an optical pickup device according to claim 4, wherein saiddetector includes an output sensor for measuring output of said lightsource as said control factor, and said controller controls said moverbased on a result measured by said output sensor.
 7. An optical diskrecording/reproduction apparatus including an optical pickup deviceaccording to claim 4, wherein said detector includes a position detectorfor detecting a position of said object lens along the optical axis assaid control factor, and said controller controls said mover based on aresult by said position detector.